The present invention relates to a method and an apparatus in a fluidized bed heat exchanger.
In particular, the present invention relates to a method and an apparatus, by which heat transfer may be adjusted in a fluidized bed heat exchanger. The apparatus includes a heat exchange chamber having a bed of solid particles, means for feeding fluidization gas into the heat exchange chamber, heat transfer surfaces in contact with the bed of solid particles, an inlet arranged in the top portion of the heat exchange chamber above the upper surface of the bed of solid particles, and a first outlet for removing solid particles from the heat exchange chamber. The method, meanwhile, typically includes steps of feeding solid particles through the inlet to the upper surface of the bed of solid particles in the heat exchange chamber, fluidizing the bed of solid particles in the heat exchange chamber fluidization gas, transferring heat by the heat transfer surfaces from the fluidized bed of solid particles, and removing solid particles from the heat exchange chamber through the first outlet.
Fluidized bed heat exchangers are generally used in various pressurized and atmospheric fluidized bed reactor systems, for example, in different combustion and heat transfer processes and chemical and metallurgic processes. Heat typically generated by combustion or other exothermic processes is recovered from solid particles by utilizing heat transfer surfaces. The heat transfer surfaces conduct the recovered heat to a medium, such as water or steam, which transfers the heat out of the reactor.
Heat transfer surfaces may be arranged in different parts of the reactor system, for example, in special heat exchange chambers, which may be a part of the reaction chamber, a separate chamber connected to the reaction chamber, or, as in circulating fluidized bed reactors, a part of the circulation system of solid particles.
In many applications of fluidized bed reactors, including in steam boilers, for example, it is important to be able to adjust the heat transfer continuously and accurately within a wide control range. Reasons for such adjustments may be a changing demand for steam production, a deviation in the fuel quality or in the feed of the fuel, or some other abnormality in the system. It may also be necessary to adjust the system to a correct operational state. Further reasons why it may be necessary to adjust the heat transfer in steam boilers have to do with the fact that heat is generally recovered at several stages, i.e., in evaporators, superheaters, economizers, and reheaters, which may need individual adjustment.
The purpose of adjusting the heat transfer efficiency in a fluidized bed reactor with respect to the processes is to maintain an optimum operational state in terms of emissions and efficiency in the reactor. Often this means that the temperature of the reactor should continue to be constant even when the heat transfer efficiency and the feed volumes of the fuel fluctuate.
When designing a heat exchange chamber, the most important considerations are a simple structure, continuous adjustability within a wide adjustment range, and minimal space requirements.
One way to adjust the heat transfer efficiency of a fluidized bed heat exchanger is to change the volume of the fluidized bed material in the heat exchange chamber so that a varying portion of the heat transfer surfaces is covered by solid particles. Such a structure is disclosed, for example, in U.S. Pat. No. 4,813,479. In the disclosed arrangement, however, an additional flow channel and an adjustment valve are required, which makes the system more complicated and increases the costs. Further, when changing the height of the bed, part of the heat transfer surfaces may be exposed to considerable erosion.
U.S. Pat. No. 5,140,950 discloses an arrangement wherein the circulation flow of hot solid particles in a circulating fluidized bed reactor is divided by a number of compartments and channels into two separate chambers, only one of which includes heat transfer surfaces. By changing the division ratio of the solid particles flowing through the various chambers, it is possible to vary the heat transfer efficiency of the heat exchanger. However, the disclosed arrangement is complicated and—in terms of space consumption—disadvantageous.
A bubbling fluidized bed is usually maintained in the heat exchange chamber where the speed of the fluidization gas may be, when using bed material with small particle size, for example, 0.1-0.5 m/s. The heat transfer efficiency of the fluidized bed heat exchanger may be varied to some extent by changing the speed of the fluidization gas. This is due to the fact that the solid particles move more vividly at high speeds of the fluidization gas than they do at low speeds, whereby the hot particles spread at high speeds efficiently throughout the entire heat exchange chamber. At high speeds, no separate cooled layers are allowed to form in close proximity to the heat transfer surfaces, which could decrease the heat transfer, nor will the hot particle flows entering the heat exchanger be passed directly from the inlet of the heat exchange chamber to the outlet without mixing with the particles in the chamber.
U.S. Pat. No. 5,425,412 discloses an arrangement in a circulating fluidized bed reactor, in which the heat exchange chamber includes separate areas for transferring particles and for heat transfer, respectively. Heat transfer efficiency is adjusted by changing the moving intensity of the particles close to the heat transfer surfaces and the mixing rate of the material by utilizing the fluidization gas velocities of different areas. By changing the mixing rate of the material, the relation between the hot particles newly flown to the chamber and the particles already cooled in the exiting particle flow is varied. In different situations, particles may be discharged through an overflow opening in the bed surface and/or through an outlet in the lower portion of the chamber. The adjustment range of the heat transfer efficiency in this kind of a heat exchange chamber may, however, remain rather limited. To avoid agglomeration and overheating of the bed due to possible after-burning, the bed of solid particles must be maintained continuously fluidized, so that the mixing rate is always fairly high. Further, due to the use of a separate transfer area, the space utilization is not optimal, since a considerable part of the heat exchange chamber is not in efficient use with respect to the heat transfer.